This is all about Gödel’s Incompleteness Theorems, Frank. It is fairly simple, once you see that the model is in reality a model-of-models.

To find the completenesses of axioms, we have to stay at a lower level, at least one below the overall level. At the overall level, axioms will lead to incompletenesses. So, never overshoot the appropriate level.

Earth & Moon are a complete system by themselves.

The Solar System is a complete system by the celestial bodies themselves.

The Milky Way is a complete system.

The Universe is not a complete system.

If we want to call to call the Universe a system, then we must call it a system of systems (in which the plural and specific systems declare at what level we can find each of the completenesses, and the singular generic overall system is where we can only find incompletenesses).

Told you, mathematically this is very simple, but you need to see the model (of models).